Silver ternary alloy

ABSTRACT

A firestain and tarnish-resistant ternary alloy of silver, copper and germanium contains from more than 93.5 wt % to 95.5 wt % Ag, from 0.5 to 3 wt % Ge, optionally 0.5 wt % Zn and the remainder, apart from incidental ingredients (if any), impurities and grain refiner, copper. In order to further protect an article made from the alloy, it may be surface treated with an alkanethiol, alkyl thioglycollate, dialkyl sulphide or dialkyl disulphide. Embodiments of the above alloy exhibit relatively low elution of copper when subjected to a simulated sweat test.

REFERENCE TO PRIOR APPLICATIONS

This application is a continuation-in-part application of U.S. patentapplication Ser. No. 10/559,092 filed Nov. 22, 2005 which claimspriority from International patent application PCT/GB2004/002317 filedJun. 1, 2004 (Publication No. WO 2004/106567) which claims priority fromUK Patent Application 03 12693.5 filed Jun. 3, 2003 and issued as apatent GB 2402399, which disclosures are herein incorporated byreference in their entirety.

FIELD OF THE INVENTION

The present invention relates to a ternary alloy of silver, copper andgermanium, to finished or semi-finished shaped articles made from thealloy, and to the use for the surface treatment of the alloy withalkanethiol, alkyl thioglycollate, dialkyl sulfide or dialkyl disulfide.

BACKGROUND TO THE INVENTION

Since ancient times it has been appreciated that unalloyed ‘fine’ silveris too soft to withstand normal use, and it has been the practice to adda proportion of a base metal to increase hardness and strength. In theUK, legislation that has existed since the fourteenth century specifiesa minimum silver content of articles for sale at 92.5% (the Sterlingstandard) or 96% (the Britannia standard), but does not specify the basemetal constituents. Experience convinced early silversmiths that copperwas the most suitable of the metals available to them. Modernsilver-sheet manufacturers generally adhere to this composition,although sometimes a proportion of copper is replaced by cadmium toattain even greater ductility. Sterling with a 2.5% cadmium content ispermitted to be used in the UK for spinning and stamping, although theuse of cadmium is becoming less widespread because when the alloy is inthe molten state fumes of cadmium are given off and are toxic. For thatreason, a specific alloy composition is produced in continental Europethat contains 93.5 wt % silver. It is sold as a sterling grade alloywith enhanced forming properties for deep drawing or spinningoperations. The high silver content reduces the hardness of the alloy,but not to a level where a finished item would be too soft and subjectto excessive damage from handling. It therefore can provide acadmium-free spinning grade, sold to perform similar forming operationsto the cadmium-containing silver grades available in the UK.

The relatively high value assigned to silver compared to the otheralloying constituents in the different grades has meant thatmanufacturers have aimed to produce their alloys as closely as possibleto the minimum legal silver content. This has also resulted in a systemin the UK of ensuring that the minimum silver content is assured bymeans of independent external verification—the Assay Office System.

In all but the largest manufacturing companies, most of the annealingand soldering required to assemble finished or semi-finished articles iscarried out with the flame of an air-gas blowtorch. The oxidising orreducing nature of the flame and the temperature of the articles arecontrolled only by the skill of the silversmith. Pure silver allowsoxygen to pass easily through it, particularly when the silver is heatedto above red heat. Silver does not oxidise in air, but the copper in asilver/copper alloy is oxidised to cuprous or cupric oxide. Pickling ofthe oxidised surface of the article in hot dilute sulphuric acid removesthe superficial but not the deeper seated copper oxide so that thesurface consists of fine or unalloyed silver covering a layer ofsilver/copper oxide mixture. The pure silver is easily permeated duringfurther heating, allowing copper located deeper below the surface tobecome oxidised. Successive annealing, cold working and picklingproduces a surface that exhibits the pure lustre of silver when lightlypolished but with heavier polishing reveals dark and disfiguring stainsknown as ‘fire-stain’ or ‘fire’. Soldering operations are much moreproductive of deep fire-stain because of the higher temperaturesinvolved. When the depth of the fire-stain exceeds about 0.025 mm (0.010inches) the alloy is additionally prone to cracking and difficult tosolder because an oxide surface is not wetted by solder so that a propermetallurgical bond is not formed.

Secondly, it is a well-known fact that with exposure to everydayatmospheric conditions, silver and silver alloys develop alustre-destroying dark film known as tarnish.

The addition of germanium to silver alloys was found to offer a solutionto both these problems. Patent GB-B-2255348 (Rateau, Albert and Johns;Metaleurop Recherche) disclosed a novel silver alloy that maintained theproperties of hardness and lustre inherent in Ag—Cu alloys whilereducing problems resulting from the tendency of the copper content tooxidise. The alloys were ternary Ag—Cu—Ge alloys containing at least92.5 wt % Ag, 0.5-3 wt % Ge and the balance, apart from impurities,copper. The alloys were stated to be stainless in ambient air duringconventional production, transformation and finishing operations, to beeasily deformable when cold, to be easily brazed and not to give rise tosignificant shrinkage on casting. They were also stated to exhibitsuperior ductility and tensile strength and to be annealable to arequired hardness. Germanium was stated to exert a protective functionthat was responsible for the advantageous combination of propertiesexhibited by the new alloys, and was in solid solution in both thesilver and the copper phases. The microstructure of the alloy was saidto be constituted by two phases, a solid solution of germanium andcopper in silver surrounded by a filamentous solid solution of germaniumand silver and copper. The germanium in the copper-rich phase was saidto inhibit surface oxidation of that phase by forming a thin GeO or GeO₂protective coating which prevented the appearance of fire-stain duringbrazing and flame annealing which results from the oxidation of copperat high temperatures. Furthermore the development of tarnish wasappreciably delayed by the addition of germanium, the surface turnedslightly yellow rather than black and tarnish products were easilyremoved by ordinary tap water. The alloy was said to be useful interalia in jewelery. However, the alloy disclosed in the above patentsuffers limitations insofar as it can exhibit large grain size, leadingto poor deformation properties and formation of large pools fromlow-melting eutectics resulting in localised surface melting when thealloy is subject to the heat of an air torch.

Furthermore, U.S. Pat. No. 6,168,071 and EP-B-0729398 (Johns) discloseda silver/germanium alloy which comprised a silver content of at least 77wt % and a germanium content of between 0.4 and 7%, the remainderprincipally being copper apart from any impurities, which alloycontained elemental boron as a grain refiner at a concentration ofgreater than 0 ppm and less than 20 ppm. The boron content of the alloycould be achieved by providing the boron in a master copper/boron alloyhaving 2 wt % elemental boron. It was reported that such lowconcentrations of boron surprisingly provided excellent grain refiningin a silver/germanium alloy, imparting greater strength and ductility tothe alloy compared with a silver/germanium alloy without boron. Theboron in the alloy inhibited grain growth even at temperatures used inthe jewellery trade for soldering, and samples of the alloy werereported to have resisted pitting even upon heating repeatedly totemperatures where in the previously suggested alloys a brieflymentioned copper/germanium eutectic in the alloy would melt. Strong andaesthetically pleasing joints between separate elements of the alloycould be obtained without using a filler material between the freesurfaces of the two elements and a butt or lap joint could be formed bya diffusion process or resistance or laser welding techniques. Comparedto a weld in Sterling silver, a weld in the above-described alloy had amuch smaller average grain size that improved the formability andductility of the welds, and an 830 alloy had been welded by laserwelding and polished without the need for grinding. It may be noted withregard to the above mentioned eutectic that although its adverse effectsare reduced by the reduction in grain size, the ability of the eutecticto form and become molten on post-formation thermal treatment isretained because that is governed by the chemical composition of thealloy rather than its crystal structure.

Silver alloy according to the teaching of GB-B-2255348 and EP-B-0729398is now commercially available in Europe and in the USA under the tradename Argentium, and the word “Argentium” as used herein refers to thesealloys. The 925 grade Argentium alloy comprises 92.5 wt % (minimum) Ag,1.1-1.3 wt % Ge, 6 ppm B, the balance being copper and impurities. Thealloy shows excellent resistance to tarnishing even under very arduousconditions. A passive layer is formed by the germanium, whichsignificantly slows the formation of silver sulphide, the main cause oftarnishing on conventional silver alloys. Even in a hydrogen sulphideatmosphere the degree and depth of tarnish is significantly lesscompared to a conventional silver alloy or a silver plated item. Thesame mechanism that creates the tarnish resistance also results in theformation of a passive layer which significantly reduces the depth of‘fire-staining’ or the ‘fire layer’ that is produced in this alloy whentorch annealing in air. Trials have shown that the depth of the‘fire-staining’ to be up to three times greater in conventional silveralloys compared to the Argentium silver alloys. This reduces the amountof polishing that the alloy requires and can result in otherconsiderable cost savings in manufacturing.

Despite the advantages of existing Argentium alloy grades, there is aneed for further improvement of the alloy with respect to its stabilityunder thermal processing and in particular to its resistance to pittingand/or sagging when heated for the purposes of annealing or joining.There is also a need for alloys that combine these favourable propertieswith hardness and resistance to tarnishing. There is a further need foralloys that on investment casting has a reduced propensity to formationof “hot short” (cracking) defects.

The need for improvement of alloys of silver, copper and germaniumcreated by a low melting eutectic phase was recognised in GB-A-2355990.That specification explained that over a germanium content range of 0.1%to 3%, it was found that the addition of tin, antimony, silicon orindium in small quantities, less than 2% by weight, brought aboutimprovement in the mechanical properties of the resultantsilver/copper/germanium alloy. For example, heat distortion such assagging of the silver/copper/germanium alloy at high solder temperatureswas reduced, the alloy could be more readily rolled further from the ascast state and the alloy did not work harden so quickly during othercold work processes. In one example, a silver/copper/germanium alloy hada germanium content of 1% and included a tin content of between 0.01 and1%. The silver/copper/germanium alloy also had an elemental boroncontent of 100 ppm or less. The addition of tin positively influencedthe distribution of the low melting point eutectics and caused areduction in coarse dendritic formations and secondary dendritic arms.Thus, excessively high concentrations of low melting point phases whichpredisposed the alloy to heat distortion were avoided and the resultantalloy was less susceptible to heat distortion at high solderingtemperatures (typically >700° C.) than the same silver/copper/germaniumalloy would have been without the tin content. The effects of usingantimony, silicon or indium in place of tin as an additive werecomparable. Additionally, beneficial effects were achievable with anadditive comprising a combination of any or all of tin, antimony,silicon or indium. There was no suggestion that the alleged advantagescould be obtained in the absence of antimony, silicon or indium, and inpractice the alleged advantages were insufficiently obtained.

As a further matter, so far as long-term tarnish resistance isconcerned, various proposals have been made for cleaning or protectingSterling silver and other known grades of silver to remove tarnishand/or to inhibit the formation of tarnish. GB-A-1130540 is concernedwith the protection of a finished surface of Sterling or Britanniasilver as a step in a production run, and discloses a process thatcomprises the steps of:

wetting a clean silver surface of an article with a solution comprising99 parts by weight of a volatile organic solvent, for exampletrichloroethylene or 1,1,1-trichloroethane and from 0.1-1.8 parts byweight of an organic solute containing a —SH group and capable offorming a transparent colourless protective layer on the silver surface,for example stearyl and cetyl mercaptan or thioglycollate;

allowing the solution to react with the surface to form such a layer andallowing the solvent to evaporate; and

washing the surface with a detergent solution, rinsing the surface withhot water and allowing it to dry. The above process is stated to providea “long-term finish” intended to last the intended shelf-life until thearticle reaches the user.

Treatments of the above kind are believed to result in the formation ofa self-assembled coating derived from the thiol compounds in which thesulphur atoms are bound onto the metal surface and the alkyl tails aredirected away from the metal surface, see U.S. Pat. No. 6,183,815(Enick). Yousong Kim et al,http://www.electrochem.org/meetings/past/200/abstracts/symposia/h1/1026.pdfreported that the adsorption of thiols onto silver proceeds through ananodic oxidation reaction that produces a shift of the open circuitpotential of the substrate metal in the negative direction or if thepotential is fixed an anodic current peak:

RSH+M(0)→RS-M(I)+H++e−(M) (M=Au or Ag)

-   -   Kwan Kim, Adsorption and Reaction of Thiols and Sulfides on        Noble Metals, Raman SRS-2000, 14-17 Aug. 2000, Xaimen, Fujian,        China, http://pcoss.org/icorsxm/paper/kuankim.pdf, also        discloses the formation of self-assembled monolayers and        discloses that alkanethiols, dialkyl sulfides and dialkyl        disulfides self-assemble on silver surfaces with aliphatic        dithiols forming dithoiolates by forming two Ag—S bonds. In        contrast, the literature on formation of alkylthiols of        germanium is relatively sparse. The reaction of alkanethiols        with Ge to form a high quality monolayer has been reported in        the context of semiconductor and nanotechnology by Han et al, J.        Am. Chem. Soc., 123, 2422 (2001). In the experiment described, a        Ge(111) wafer is sonicated in acetone to dissolve organic        contaminants and immersed in concentrated HF to remove residual        oxide and produce a hydrogen-terminated surface, after which the        wafer is immersed in an alknaethiol solution in isopropanol,        sonicated in propanol and dried.

SUMMARY OF THE INVENTION

Although GB-B-2255348 discloses a range of silver content from Sterlingto Britannia grades, as previously explained a skilled person would notcontemplate using a silver content above the legal minimum for theintended grade because of the cost of the silver. He is not given by thedisclosure of that patent or that of the subsequent patents relating tothe ternary alloys any incentive to adopt intermediate silver contents.However, it has now been found that silver-copper-germanium ternaryalloys having silver contents between those of the Sterling andBritannia grades possess valuable properties that facilitate bothcasting, welding and other thermal treatments used in the manufacture ofsilver articles.

In particular, the applicants have become aware of the desirability ofreducing or avoiding the formation and/or melting of the above mentionedbinary copper-germanium eutectic which melts at 554° C. During theproduction of e.g. 925 Argentium silver alloys, the formation of thisphase can be avoided by careful control of the casting conditions sinceunder equilibrium cooling conditions the crystallisation is complete atbelow 640° C. However, this binary phase can create problems duringsubsequent thermal treatment of the alloys, e.g. using brazing alloyswhich typically have melting points in the range 680-750° and torchannealing which typically involves heating a workpiece to a dull redheat at 700-750° C. On heating the workpiece to or beyond thesetemperatures incipient melting occurs with a small amount of materialcorresponding to this binary phase becoming molten while the bulkremains stable. When the workpiece returns to ambient temperature,porosity develops where the alloy has liquefied. This contributesbrittleness and e.g. as noted in GB-B-2255348 there is a tendency forthe alloy to sag when heated for joining or annealing operations.Although the use of the boron grain refiner of U.S. Pat. No. 6,168,071and EP-B-0729398 significantly reduces the pitting and saggingconsequent on formation and melting of the binary eutectic, theformation and melting of that eutectic is, as previously mentioned, noteliminated and there is still scope for the further development of theternary alloy to improve its pitting and sagging properties. Byincreasing the silver content above the level for Sterling but less thanthat for Britannia it is possible to produce an alloy in which the abovebinary eutectic either does not form or gives rise to reduced problemsin subsequent heat treatment. This provides alloys with a much greaterinherent stability under thermal processing. The germanium additionprevents the reduction in hardness that would be seen in a silver-copperalloy of this composition. The alloy also shows resistance totarnishing, even under very arduous test conditions.

The invention therefore provides a ternary alloy of silver, copper andgermanium containing from more than 93.5 wt % to 95.5 wt % Ag, from 0.5to 3 wt % Ge, 1-40 ppm of B, optionally 0.5 wt % of any of Zn, Cd andSn, optionally 0.1-1 wt % Si, and the remainder, apart from impuritiescopper, wherein the weight ratio of Cu to Ge is from 4:1 to 3:1.

An embodiment of the above mentioned alloys has Ag 93.5-94 wt % e.g.93.7 wt %. Another typical alloy that has been found to be suitablecontains about 94.5 wt % Ag, about 4.3 wt % Cu and about 1.2 wt % Ge. Inthe above alloy the weight ratio of Cu to Ge is about 3.6:1 whereas inthe existing 925 grade Argentium the ratio can be from 5.8:1 (1.1 wt %Ge) to 4.8:1 (1.3 wt % Ge).

The applicants consider that it is the reduction in the Cu:Ge weightratio to values <4.8:1 e.g. <4.7:1 e.g. <4.5:1 that is responsible forthe reduced thermal processing problems, the CuGe eutectic either notforming or forming in a significantly reduced amount during post-meltthermal processing. In particular the ratio is advantageously from 4.5:1to 3:1 e.g. 4:1 to 3:1, preferably about 3.5:1. Above 4:1 the alloy ismore likely to exhibit firestain, whereas below 3:1 the high germaniumcontent gives rise to formability problems. The applicants are aware ofa ζ-phase which is of hexagonal close packed Cu₅Ge and which forms theeutectic and a less harmful face centred cubic phase Cu₃Ge. Controllingthe compositional ratio of the copper and germanium to around 3:1promotes formation of the Cu₃Ge phase c. A slight excess of copper aboveits 3:1 Cu:Ge ratio allows the formation of a silver-rich a solidsolution of silver and copper, typical of the binary+silver-copperalloy. However, the optimum level of germanium is 1.2-1.5%, so to enablethis ratio between copper and germanium to be achieved the silvercontent of the alloy must be raised above the level for sterling silver(92.5%) By complying with these limitations it is possible to avoid theformation of the eutectic composition and the ζ-phase with its hcpcrystals structure which is well documented to be a poor crystalstructure for formability.

The increase in the silver level within the alloy to control crystalstructure, rather than controlling the cooling rate of the molten alloy,is a novel solution to the management of the crystal structure of thisrange of silver alloys, and hence resultant physical properties of thealloy. It is contrary to the conventional best practice within thesilver manufacturing industry, where silver content is very tightlycontrolled to reduce costs.

In an experiment, strips of three different germanium-containing silveralloys of identical size and each of thickness 0.9 mm were placed on topof steel cotter pins on a heatproof tile, and heated by means of anatural gas/compressed air torch to a temperature in the range 745-778°C. sufficient to melt a “hard” silver solder. In each sample boron waspresent in an amount effective for grain refinement but <10 ppm. Thesamples were evaluated visually as to whether they had remained straightor had sagged. Results are shown below, amounts being wt %:

Ag Cu Ge Zn Cu:Ge Sagging 93.0 5.8 1.2 n/a 4.83 yes 93.5 5.3 1.2 n/a4.41 no 93.5 4.8 1.2 0.5 4.0 no

The above results show that between a silver content of 93.0 and 93.5and a Cu:Ge ratio of 4.8 to 4.4 the thermal stability properties of thealloy change from poor to good, and that thermal stability is compatiblewith addition of 0.5 wt % zinc.

In embodiments of the above alloy, preferred Ag contents range fromabout 94.0 to about 95.5 wt %, lower values being preferred for reducingthe expense of the silver used. It has been found, surprisingly, that ifthe Ag content is increased to 96 wt % it is difficult to avoidfirestain even at high Ge contents. As regards Ge, contents of from 1.0to 2.0 wt % are preferred. Below 1.0 wt % Ge, consistent resistance tofirestain and tarnish may not be obtained, whereas above 2 wt % Ge thereis an increasing risk of embrittlement of the alloy. Furthermore, Ge isexpensive and its expense makes it desirable to reduce its content to aminimum. The applicants have found that consistent resistance tofirestain and tarnish are obtained at Ge contents of from 1.1 to 1.3 wt%. The alloy will preferably further comprise boron in an amounteffective for grain refinement, typically 1-40 ppm and preferably 5-10ppm. Excessive amounts of boron may give rise to boron hard spots, butin the case of alloys supplied for casting it will often be desirable toincorporate relatively large amounts of boron to compensate for losseson re-melting.

The alloy may contain one or more incidental ingredients known per se inthe production of silver alloys in amounts that are not detrimental tothe mechanical strength, tarnish resistance and other properties of thematerial. For example, zinc may be added e.g. in an amount of about 0.5wt % to reduce the melting point of the alloy, to add whiteness, to actas a copper substitute, as a deoxidant and to improve the fluidity ofthe alloy. Cadmium may also be added in similar amounts although its useis presently not preferred. Tin may be added, typically in an amount of0.5 wt %. Indium may be added in small quantities e.g. as a grainrefiner and to improve the wettability of the alloy. Silicon may also beadded in e.g. amounts of 0.1 to 1 wt %.

Embodiments of the above alloys exhibit low copper elution. Experimentshave been carried out using specimens with a range of different silveralloys with silver concentrations between 93.5% and 97.3%, in each casewith germanium at 1.3% and with boron at about 4 ppm. These alloys werethen subjected to a copper elution sweat test based on BS EN 1911:1999in which the samples are exposed to synthetic sweat for one week. Theelution rate of copper is then quantified by spectrophotometric analysisof the liquid. The results were as follows, the elution rate beingmeasured in μg of copper/cm²/week and weight % Ag being as made up bythe experimentalist. Made up weights are preferred in the context ofprecious metals since the practice in the industry which was followed inthis case is to weigh out constituents to at lease four significantfigures: assay figures are also available but silver content appears tohave been systematically overstated. Corresponding rates are also givenfor conventional sterling silver.

wt % Ag Elution rate 93 13.1 94 4.8 95 4.49 96 4.24 97 0.96 Sterling5.84

The samples at 94, 95 and 96 wt % Ag were made up without added boron.Since boron as grain refiner improves the microstructure of the alloy,it is likely that even lower elution rates would have been observed inboron-containing alloys. It will be noted that there is a dramaticchange in elution rate between 93.5 wt % Ag and 94.1 wt % Ag, where theelution rate falls from a value more than twice that for sterling silverto a value less than that for sterling silver.

A reason why it is feasible to reduce the copper content of the alloy toimprove physical properties and reduce copper elution compared tostandard Argentium alloys is because of the unique hardening propertiesof the AgCuGe system. Hardening can occur either by slow cooling aloneor by low temperature baking which is advantageous because quenching anyred hot silver alloy into cold water will always lead to cracking andsolder joint failure. We have observed a surprising difference inproperties between conventional sterling silver alloys and other silveralloys of the Ag—Cu family on the one hand and silver alloys of theAg—Cu—Ge family on the other hand. Gradual cooling of e.g. the binarySterling-type alloys results in coarse precipitates and littleprecipitation hardening, whereas gradual cooling of Ag—Cu—Ge alloys(including those containing the further additives and incidentalingredients set out above) results in One precipitates and usefulprecipitation hardening, especially in those embodiments where thesilver alloy contains an effective amount of grain refiner e.g. boron.

Experimental evidence has shown that Ag—Cu—Ge alloys of Ag content 93.5wt % and above become precipitation hardened following cooling from amelting or annealing temperature by baking at e.g. 200° C.-400° C. andthat baking the alloy can achieve a hardness of 65 or above, preferably70 HV or above and still more preferably 75 DV or above which is equalto or above the hardness of standard sterling silver used to makejewelery and other silverware. These advantageous properties arebelieved to be the result of the combination of Cu and Ge in the silveralloy and are independent of the presence and amounts of Zn or otherincidental alloying ingredients.

Addition of germanium to sterling silver changes the thermalconductivity of the alloy compared to standard sterling silver. TheInternational Annealed Copper Scale (IACS) is a measure of conductivityin metals. On this scale the value of copper is 100%, pure silver is106%, and standard sterling silver 96%, while a sterling alloycontaining 1.1% germanium has a conductivity of 56%. The significance isthat the Argentium sterling and other germanium-containing silver alloysdo not dissipate heat as quickly as standard sterling silver or theirnon-germanium-containing equivalents, a piece will take longer to cool,and precipitation hardening to a commercially useful level (e.g. toabout Vickers hardness 70 or above, preferably to Vickers hardness 110or above, more preferably to 115 or above) can take place during naturalair cooling or during slow controlled air cooling. Silver alloy of Ag973 parts per thousand and containing about 1.0 wt % Ge, balance copper,has been successfully precipitation hardened by gradual air cooling froman annealing temperature, and it is believed that Ag—Cu—Ge alloys withsilver content above this level are also precipitation hardenable

The benefit of not having to quench to achieve the hardening affect is amajor advantage of the present silver alloys. There are very few timesin practical production that a silversmith can safely quench a piece ofnearly finished work. The risk of distortion and damage to solderedjoints when quenching from a high temperature would make the process notcommercially viable. In fact standard sterling can also be precipitationhardened but only with quenching from the annealing temperature and thisis one reason why precipitation hardening is not used for sterlingsilver.

In order to distinguish the operations of annealing and precipitationhardening (which are regarded as distinct by silversmiths) annealingtemperatures may be defined to be temperatures above 500° C., whereasprecipitation hardening temperatures may be defined to be in the range150° C.-400° C., the lower value of 150° C. permitting embodiments ofthe alloys of the invention to be precipitation hardened in a domesticoven.

The alloy may be produced by continuous casting. The initial castingconditions may be as for an equivalent silver-copper grade except thatthe germanium addition will give solidus and liquidus temperaturesapproximately 15° C. lower than the equivalent silver-copper alloys. Thegermanium content also alters the emissivity of the alloy. This willaffect the rate at which heat can be removed from the die (if continuouscasting) or standing time (if static casting). It may also be desirableto re-calibrate optical (infra-red) pyrometers when casting the presenttertiary alloys. This is because the germanium content gives the alloysa different emissivity. Typically the sensors would give a much lowerreading than the actual temperature if they were not adjusted. Duringbillet production from continuously cast slabs it is a requirement toremove the surface layer, which has been in contact with the die. Thisis the layer that acted as the starting point for solidification of themetal and it contains the most impurities. To remove this layer aminimum of 0.01 mm should be removed from each side of the cast slab byeither a mechanical or an abrasive technique.

To produce a sheet with an internal structure suitable for further useby the silversmith a rolling regime which contains a minimum of two 50%reductions and two anneals is recommended. This will remove the‘as-cast’ grain structure and prevent any orange peel effects when thesheet is being formed by the silversmith. As an example, to producesheet at 2.5 mm thick the following would be the minimum rollingrequirements:

Starting size  10 mm thick Roll to 5.0 mm thick (50% reduction) AnnealRoll to 2.5 mm thick (50% reduction) Anneal

When cross-rolling to increase the width of sheets, at the end of thecross-rolling regime the sheet should be annealed prior to thecommencement of the normal rolling schedule.

When drawing the alloy into wire, the required drawing sequence dependson the internal grain structure of the starting material. This isbecause the wire could be from two possible sources, either a cold orwarm working operation (e.g. extrusion) or from an ‘as-cast’ wire size(e.g. a ‘mini’ casting system). For material that has come from aprevious cold worked source the only constraint is that the material hasa minimum of 25% cold work prior to each anneal. This will preventexcessive grain growth. A maximum of 60% cold work between each annealis recommended. For example, the following work procedures would applyto wire from a previously cold worked source:

Starting size   6 mm diameter Draw to 5.2 mm diameter (25% reduction) OrStarting size   6 mm diameter Draw to 3.8 mm diameter (60% reduction)Anneal Draw to 2.4 mm diameter (60% reduction) Anneal etc.

For material that is from a cast source then a drawing sequenceinvolving two reductions of a minimum of 50% and two anneals recommendedto give a grain size suitable for further work by the silversmith. Workprocedures would be similar to those given above.

When annealing the alloy, it is important that the furnace gas, althoughprotective, does not deplete the surface layer of germanium, as thiswill reduce the tarnish resistance of the alloy and its resistance to“fire stain”. It is the ability of the surface layer of germanium toform a germanium oxide which then acts as a barrier preventing anyfurther penetration of the oxide layer or build up of tarnishingproducts. For this reason a furnace atmosphere based on cracked ammoniais not recommended. To prevent depletion of the germanium from thesurface of the alloy the presence of a small amount of oxygen or aslightly “wet” furnace atmosphere is beneficial. Typically the furnaceatmosphere should contain approximately 0.1-0.5% oxygen and have a dewpoint of 20-40° C. The exact balance of these values depends on the typeof furnace being used. It is important that the balance is not set tofar in the opposite direction as this may result in oxidation of thecopper content of the alloy. The annealing temperature may be within therange 620-650° C. and should preferably not exceed a maximum temperatureof 650° C. The annealing time for this temperature range is 30 to 45minutes.

As regards cleaning procedures, although GB-A-1130540 was alleged toprovide a long-term finish, in the inventor's experience this type oftreatment does not fully solve the difficulties created by tarnish inthe period between manufacture and supply to the ultimate purchaser oruser and suffers from a number of shortcomings. Although a silverproduct might arrive at the retailer in an untarnished state, it waslargely the result of the wrapping applied by the manufacturer, whichprotected the article from air. Once the wrapping was removed and thearticle was displayed in a retail environment such as a display case ina hotel where it was subject to ambient air and the heat of artificiallighting, an article of conventional Sterling silver would requirere-polishing after one week and after two weeks would normally be sotarnished as to be un-saleable. At an exhibition, the life of an articleon display before significant tarnish sets in may be as short as 3-4days. Re-polishing produces wear and fine handling scratches, so thatunless the article can be sold quickly it looses its pristineappearance. The need to polish display silver at frequent intervals addsto the labour cost of a jeweler or other retail establishment, whosemanagement take the view that its staff should be employed to sellproducts and not to clean stock. Tarnish at point of sale is therefore aserious problem that reduces the willingness of those in thedistribution chain to stock and display silver products, and which hasnot yet been adequately solved.

When the product reaches the ultimate purchaser, it is of coursedesirable that the task of tarnish removal should be made as infrequentand undemanding as possible.

It has now been found that an alkanethiol, alkyl thioglycollate, dialkylsulphide or dialkyl disulphide can be used for the surface treatment ofthe above described alloys, preferably so as to reduce or further reducetarnishing of the alloy such that a sample can be subjected to hydrogensulphide gas above a 20% solution of ammonium polysulphide for at least30 minutes and typically 45-60 minutes while retaining a generallyuntarnished appearance.

The invention also therefore includes the use of an organic compoundcontaining —SH or —S—S— bonds e.g. a C₁₂-C₂₄ alkanethiol, alkylthioglycollate, dialkyl sulphide or dialkyl disulphide in thepreparation of a tarnish inhibitor for an article of a thesilver-copper-germanium alloy described above.

The invention further provides an alloy of silver as described above, ora shaped article formed of said alloy, that has been treated with aC₁₂-C₂₄ alkanethiol, alkyl thioglycollate, dialkyl sulphide or dialkyldisulphide.

In an accelerated tarnish test in which the article is subject tohydrogen sulphide gas from the ammonium polysulphide solution abovewhich it is suspended at a height of e.g. 30 mm corresponds to a periodof a year or more in a retail environment where an article is on displayand exposed to ambient atmosphere and may be subject to elevatedtemperatures. It is the combination of the protective function of thegermanium content of the alloy with the further protection from theorgano-sulphur compound that is believed to be responsible for theobserved increase in tarnish resistance. The period during which thearticle retains its untarnished appearance under these severe conditionsmay be three or more times the corresponding period for an article thathas not been treated with an organo-sulphur compound, which isunexpected because the same accelerated tarnish test carried out underthe same conditions on a conventional Sterling silver article notcontaining protective germanium does not reveal a significant increasein untarnished lifetime between its untreated and organo-sulfur treatedstates.

Accelerated tarnishing tests with Argentium Sterling using ammoniumpolysulphide have been reported by the Society of American Silversmiths,see

-   -   http://www.silversmithing.com/largentium4.htm

and in a comparative test the Argentium Sterling remained untarnishedafter one hour whereas conventional Sterling became tarnished after lessthan 15 minutes. However, in this test 0.5 ml of 20% ammoniumpolysulfide solution is mixed with 200 ml of distilled water, so thatthe test is greatly less severe than when samples are exposed to the 20%solution itself. In WO 02/095082, samples were suspended above 20%ammonium polysulphide, but the exposure times were relatively short, andonset of yellowing was reported for Ag—Cu—Ge alloys after 3-5 minutesexposure. Other tests reported in that specification involve placingsamples in a desiccator containing flowers of sulphur and calciumnitrate and are less severe than the ammonium polysulphide test.

As protective agent there may be used a compound containing a long chainalkyl group and a —SH or —S—S— group, e.g. an alkanethiol, dialkylsulfide or dialkyl disulfides in which the chain is preferably at least10 carbon atoms long and may be C₁₋₁₂—C₂₄. The —SH or —S—S— compoundsthat many be used include straight chain saturated aliphatic compoundscontaining 16-24 carbon atoms in the chain, for example stearylmercaptan, cetyl mercaptan (octadecyl mercaptan) and stearyl and cetylthioglycollates whose formulae appear below.

Stearyl mercaptan is a white to pale yellow waxy solid that is insolublein water. The protective agent may be used in solution in a solvent e.g.a non-polar organic solvent such as an alcohol e.g. methyl or ethylalcohol, a ketone e.g. acetone or methyl ethyl ketone, an ether e.g.diethyl ether, an ester e.g. n-butyl acetate, a hydrocarbon, ahalocarbon e.g. methylene chloride, 1,1,1-trichloroethane,trichloroethylene, perchloroethylene or HCFC 141b. The protective agentmay comprise 0.1-1 wt % of the solvent. Solvents based on n-propylbromide are presently preferred on the ground of the short atmosphericlife of that compound, its relatively low toxicity compared to otherhalocarbons, its favourable chemical and physical properties and itsboiling point, specific heat and latent heat of vaporization.

U.S. Pat. No. 5,616,549 discloses a solvent mixture comprising: 90percent to about 96.5 percent n-propyl bromide; 0 percent to about 6.5percent of a mixture of terpenes, the terpene mixture comprising 35percent to about 50 percent cis-pinane and 35 percent to about 50percent trans-pinane; and 3.5 percent to about 5 percent of a mixture oflow boiling solvents, the low boiling solvent mixture comprising 0.5percent to 1 percent nitromethane, 0.5 percent to 1 percent 1,2-butyleneoxide and 2.5 percent to 3 percent 1,3-dioxolane. The solvent mixturehas the following advantages:

(i) it is properly stabilized against any free acid that might resultfrom oxidation of the mixture in the presence of air, from hydrolysis ofthe mixture in the presence of water, and from pyrolysis of the mixtureunder the influence of high temperatures;

(ii) it is non-flammable and non-corrosive;

(iii) the various components of the solvent mixture are not regulated bythe U.S. Clean Air Act; and

(iv) none of the various components of the solvent mixture are knowncancer causing agents (i.e., the various components are not listed byN.T.I., I.A.R.C. and California Proposition 65, nor are they regulatedby OSHA). Moreover, the solvent mixture has a high solvency with akauri-butanol value above 120 and, more preferably, above 125. Inaddition, the solvent mixture has an evaporation rate of at least 0.96where 1,1,1-Trichloroethane=1. Upon evaporation, the solvent mixtureleaves a non-volatile residue (NVR) of less than 2.5 mg and, morepreferably, no residue. Solvents made in accordance with the abovepatent are available from Enviro-Tech International, Inc of MelrosePark, Ill., USA under the trade name EnSolv.

The surface treatment may be carried out after the manufacturing stagesfor a shaped article made of the alloy have been completed. The articlemay be of flatware, hollowware or jewellery. Fabrication steps mayinclude spinning, pressing, forging, casting, chasing, hammering fromsheet, planishing, joining by soldering brazing or welding, annealingand polishing using buffs/mops and aluminium oxide or rouge. An articleto be treated may be de-greased ultrasonically in a treatment bath,dipped into a bath containing the treatment agent e.g. 1 wt % stearylmercaptan in solvent e.g. EnSolv, rinsed in one or more baths of thesolvent and allowed to dry by evaporation. The solvent should leave noor substantially no residue, so that subsequent washing with water oraqueous solvents should be unnecessary and the article can be allowed todry. The article may then be packed for delivery into the distributionchain. This may include wrapping the article in one or more protectivesheets, placing it in a presentation box, and wrapping the presentationbox in a protective wrapping e.g. of heat-shrunk plastics film. Articleswhich have been treated with an organic compound containing —SH or —S—S—groups as aforesaid and packaged should not only reach their point ofsale in good condition but should if displayed e.g. on a shelf or in acabinet for an extended period, expected to be at least 6 months andpossibly 12 months or more, remain without development of significanttarnish.

For many purposes, e.g. light industrial applications, it may bepreferred to carry out the anti-tarnish treatment using a predominantlyaqueous solvent system. For this purpose, the protective agent may bedissolved in a water-immiscible organic solvent, for example a solventbased on n-propyl bromide, the resulting solution may be mixed with arelatively concentrated water-based soap or detergent composition whichacts as a “carrier”, after which water is added to the resulting mixtureto provide an aqueous treatment dip or combined degreasing and treatmentsolution. Thus an aqueous dip has the advantages that a solventdegreasing system is not necessary, the dip is easily made and may beused cold, all areas of immersed articles can come into contact with thestearyl mercaptan or other treatment agent, the present alloy onlyrequires 2 minutes in the dip, rinsing and drying of articles are madeeasy as water droplets are repelled from the surface of the polishedalloy, and the dip can be easily used in a manufacturing environmentbefore articles are sent to retailers.

Preferred water-based detergents may be based on anionic, alkoxylatednon-ionic or water-soluble cationic surface active agents or mixtures ofthem and preferably have a pH at or close to 7. Anionic surfactants maybe based on alkyl sulphates and alkyl benzene sulphonates, whoseharshness on prolonged skin exposure may be reduced by the co-presenceor use of alkyl ethoxy sulphates (U.S. Pat. No. 3,793,233, Rose et al.;U.S. Pat. No. 4,024,078 Gilbert; U.S. Pat. No. 4,316,824 Pancherni).Other known surfactants e.g. betaines may also be present, see e.g. U.S.Pat. No. 4,555,360 (Bissett). A suitable formulation containing 5-15 wt% non-ionic surfactants and 15-30 wt % anionic surfactants is availablecommercially in the UK under the trade name Fairy Liquid (Proctor &Gamble).

An aqueous liquid may also be made by dissolving the treatment agent ina non-organic solvent and adding a relatively concentrated aqueousdetergent liquid, for example undiluted Fairy Liquid. This provides adetergent liquid that has a number of advantages: the soapy liquid iseasily made, the liquid is easily applied to the articles of the presentalloy with a damp sponge/cotton wool/cloth etc, the liquid and latherenables the stearyl mercaptan or other treatment agent to get into thoseawkward areas on an article where a cloth may not be able to reach,rinsing and drying of articles are made easy as water droplets arerepelled from the surface of the polished silver, the process can beeasily used in a manufacturing environment before articles are sent toretailers and can also be easily used in a retail or domesticenvironment.

The articles may alternatively simply be polished with a polishcontaining 1-5 wt % of the organo-sulphur compound e.g. stearylmercaptan together surfactants and a cleaning agent e.g. diatomaceousearth in a solvent. As a further alternative, they may be simplypolished with a cloth impregnated with the sogano-sulphur compound e.g.stearyl mercaptan. The advantages of a cleaning cloth are that it iseasily manufactured, can be easily used in a retail or domesticenvironment and is good for general upkeep of articles of the presentalloy (if required).

The invention will now be further described, by way of illustrationonly, with reference to the following examples. Throughout the examples,the term “enhanced tarnish resistance” of samples treated with stearylmercaptan refers to the comparison with samples of Argentium Silverwhich have not had any treatment except for polishing and cleaning inEnSolv 765.

EXAMPLE 1 Production and Properties of Continuously Cast Alloy

A ternary silver-copper-germanium alloy (Ag=94.5 wt %, Ge=1.2 wt %,Cu=4.1 wt %, B=0.0008 wt % (8 ppm) is prepared by melting silver, copperand germanium together at 1050° C. under an atmosphere of nitrogen, andadding the boron as a copper-boron master alloy at the last possiblemoment. The molten mixture is then continuously cast into strip of width50 mm and of thickness 10 mm, after which an impurity-rich surface layerof at least 0.1 mm is scalped off the surface of the cast strip by ametal planer. The cast strip is then cold rolled to 5 mm thickness,annealed at 620-650° C. for 30-45 minutes in a slightly wet protectivegas atmosphere containing 0.1-0.5% oxygen and having a dew point of20-40° C., these conditions being selected to promote formation of GeO₂without oxidizing copper to copper oxides, then subjected to a secondrolling to a thickness of 2.5 mm and a second annealing, after which the“as cast” grain structure has been substantially removed and the sheetcan be formed by a silversmith without exhibiting objectionableorange-peel effects.

The rolled annealed sheet has a measured hardness of 64 HV which iscomparable to values for Sterling silver. A sample of the rolled sheetcan be annealed by heating to about 600° C. followed by quenching, andthe procedure can be repeated six times, after which the sample is ingood condition apart from slight edge cracking and exhibits nofire-stain. This behaviour differs from that of normal Sterling silverwhich tends to crack if quenched from red heat, and also from that of925 Argentium whose low-melting (554° C.) ternarysilver-copper-germanium eutectic would be liquid at the quenchingtemperature and would cause the alloy to shatter when quenched.

EXAMPLE 2 Production and Properties of Investment Cast Strip

The molten alloy of Example 1 is formed into strip by investmentcasting. The resulting strip is substantially free of “hot short”defects and brittleness, and has a hardness of 63.5 HV.

EXAMPLE 3 Solvent Dip Cleaning Solvent Degreased Samples

Solutions are made up containing stearyl mercaptan (0.1, 0.5 and 1.0gram) in EnSolv 765 (100 ml). Samples of the rolled annealed ternaryalloy sheet of Example 1 which have been polished and ultrasonicallydegreased in EnSolv 765 for 2 minutes are each immersed in one of thestearyl mercaptan solutions for periods of 2 minutes, 5 minutes and 15minutes. The samples are then buffed with clean cotton wool.

-   -   In order to evaluate tarnish resistance, the alloy samples are        supported on a glass slide in a fume cupboard about 25 mm above        the surface of 20% ammonium polysulphide solution so as to be        exposed to the hydrogen sulphide that arises from that solution.        All of the samples demonstrate good tarnish resistance during a        one-hour test, with very slight yellowing after 45 minutes        exposure to the hydrogen sulphide. The light film on the samples        is easily removed with a cleaning cloth impregnated with stearyl        mercaptan.

By way of comparison, a standard Sterling silver sample starts todiscolour as soon as it is subjected to the above test and after onehour had forms a heavy black tarnish which can not be removed with acleaning cloth impregnated with stearyl mercaptan.

EXAMPLE 4 Effect of Post-Treatment Solvent Cleaning

Example 3 is repeated for the ternary alloy samples except that insteadof buffing with cotton wool after the mercaptan treatment, the samplesare ultrasonically degreased in EnSolv 765 for 2 minutes. The samplesare then tarnish tested as described in Example 3 and all show enhancedtarnish resistance. The ability of the protective effect of the stearylmercaptan treatment to survive ultrasonic cleaning in EnSolv suggeststhat the tarnish resistance is achieved by a surface reaction involvingthe stearyl mercaptan and possibly the germanium in the present alloy,and not by formation of a grease or oil layer on the surface of thepresent alloy.

EXAMPLE 5 Aqueous Dip Application Solvent Degreased Samples

An anti-tarnish treatment solution is prepared using the followingingredients:

Stearyl mercaptan 1 g EnSolv 765 5 ml Detergent (Fairy Liquid) 40 mlDe-ionised water 100 ml

The Stearyl Mercaptan is dissolved into the EnSolv 765 after which theresulting solution is mixed with detergent (Fairy Liquid) and dilutedwith water to provide an aqueous dip. Samples of the ternary alloy ofExample 1 are polished and ultrasonically degreased in EnSolv 765 for 2minutes, immersed into the above aqueous dip for 2 minutes at ambienttemperatures and then rinsed under running tap water It may be noted thewater is immediately repelled from the polished surface, which leavesthe samples dry. Samples may be tarnish tested as described in Example 3and all show enhanced tarnish resistance.

EXAMPLE 6

Direct “Sponge” Application—Neat Detergent Solutions (solventdegreased/aqueous degreased samples)

The following solutions are prepared:

-   -   1 gram stearyl mercaptan    -   5 ml EnSolv 765    -   40 ml detergent (Fairy Liquid) (Preferred quantities)    -   1 gram stearyl mercaptan    -   5 ml EnSolv 765    -   40 ml soap (liquid hand soap)

The stearyl mercaptan is initially dissolved into the EnSolv. Thedetergent is then mixed into the solutions. Samples of the rolledannealed alloy of Example 1 are polished and ultrasonically degreased inEnSolv 765 for 2 minutes. The stearyl mercaptan/EnSolv/detergentsolutions are then directly applied to the surface of the samples usingdamp cotton wool and massaged into lather. The samples are then rinsedunder running tap water. In each case, it is noted that water isrepelled off of the polished surface, leaving the samples dry. Samplesare tarnish tested as in Example 3 by being exposed to neat ammoniumpolysulphide solution over a period of 1 hour. They all show enhancedtarnish resistance. The above direct “sponging” method for applying theStearyl Mercaptan is tested on ternary alloy strip samples degreased ina 2% Fairy Liquid aqueous solution. Enhanced tarnish resistance is againachieved.

1. An alloy, comprising: from 93.5 wt % to 95.5 wt % of silver; from 0.5to 3 wt % of germanium; 1-40 parts per million of boron; and theremainder, apart from impurities, being copper; the weight ratio ofcopper to germanium being from 4:1 to 3:1; and the alloy being resistantto the development of porosity and brittleness, the development of hotshort cracking defects when investment cast, the development of cracksor shattering on annealing and quenching and the development of cracksand sagging when heated for joining or torch annealing.
 2. The alloy ofclaim 1, wherein the weight ratio of copper to germanium is about 3.5:1.3. The alloy of claim 1, which comprises from 1.0 wt % to 1.5 wt % ofgermanium.
 4. The alloy of claim 1, which comprises about 94.5 wt % ofsilver, about 4.3 wt % of copper and about 1.2 wt % of germanium.
 5. Thealloy of claim 1, which comprises 5-20 ppm of boron.
 6. The alloy ofclaim 1, wherein said alloy is a finished or semi-finished shapedarticle.
 7. The alloy of claim 6, wherein said finished or semi-finishedshaped article is casting.
 8. The alloy of claim 6, wherein saidfinished or semi-finished shaped article is produced or partiallyproduced from a sheet or a strip.
 9. The alloy of claim 1, whichcomprises more than 93.5 wt % of silver.
 10. The alloy of claim 1, whichfurther comprises 0.5 wt % of zinc.
 11. An alloy, comprising: from 93.7wt % to 95.5 wt % silver; from 1.0 to 1.5 wt % germanium; 1-40 parts permillion of boron; and the remainder, apart from impurities, beingcopper; the weight ratio of copper to germanium being from 4.5:1 to 3:1;and the alloy being resistant to the development of porosity andbrittleness, the development of hot short cracking defects wheninvestment cast, the development of cracks or shattering on annealingand quenching and the development of cracks and sagging when heated forjoining or torch annealing.
 12. The alloy of claim 11, which furthercomprises 0.5 wt % of zinc.
 13. The alloy of claim 11, comprising about93.7 wt % of silver Ag, about 5.1 wt % Gu of copper and 1.2 wt % ofgermanium.
 14. A ternary alloy of silver, copper and germanium,comprising: from more than 93.5 wt % to 95.5 wt % of silver; from 0.5 wt% to 3 wt % of germanium; and a remainder comprising, apart fromincidental ingredients in amounts that are not detrimental to mechanicalstrength and tarnish resistance of said ternary alloy, impurities andgrain refiner and copper, said ternary alloy being resistant todeveloping of hot short cracking defects when investment cast andresistant to developing of cracks and sagging when heated for joining ortorch annealing.